1. Field of the Invention
This invention relates to thixotropic silicone rubber compositions for injection molding.
2. Background Art
As compared with metals and plastics, rubber materials generally have good shock absorbing characteristics. Among others, silicone rubber finds a wider variety of applications because of its heat resistance, weather resistance and electrical insulation. However, rubber materials, which absorb impact energy mainly by viscoelastic strains, must have a certain thickness in the impact force direction and hence, a certain weight. Since weight reduction is the trend in various fields including transportation vehicles, business machines and household appliances, there is a demand for a material which provides sufficient elasticity despite light weight and allows a number of parts to be molded.
One effective method for reducing the weight of rubber material is to admix a fine hollow filler with silicone rubber. Although a substantial weight reduction is achieved, there often occurs a separation phenomenon that the hollow filler becomes afloat in a short time due to the light weight of the hollow filler and hence, the substantial difference in specific gravity between the rubber base and the hollow filler. Particularly in an injection molding system of continuously performing molding after loading of the molding material, a series of molded parts vary in specific gravity due to the phenomenon of filler balloons separating with time. The cured rubber parts have differing weights.
There is a desire to have an injection molding silicone rubber composition loaded with a hollow filler which remains stable while preventing separation of the filler balloons.